The present invention relates to apparatus and methods for monitoring a parameter in the blood of a living organism.
Certain constituents in the blood affect the absorption of light at various wavelengths by the blood. For example, oxygen in the blood binds to hemoglobin to form oxyhemoglobin. Oxyhemoglobin absorbs light more strongly in the infrared region than in the red region, whereas hemoglobin exhibits the reverse behavior. Therefore, highly oxygenated blood with a high concentration of oxyhemoglobin and a low concentration of hemoglobin will tend to have a high ratio of optical transmissivity in the red region to optical transmissivity in the infrared region. The ratio of transmissivities of the blood at red and infrared wavelengths can be employed as a measure of oxygen saturation.
This principle has been used heretofore in oximeters for monitoring oxygen saturation of the blood in the body of a living organism as, for example, in patients undergoing surgery. As disclosed in U.S. Pat. No. 4,407,290, oximeters for this purpose may include red light and infrared light emitting diodes together with a photodetector. The diodes and photodetector typically are incorporated in a probe arranged to fit on a body structure such as an earlobe or a fingertip, so that light from the diodes is transmitted through the body structure to the photodetector. The infrared and red light emitting diodes are switched on and off in alternating sequence at a switching frequency far greater than the pulse frequency. The signal produced by the photodetector includes alternating portions representing red and infrared light passing through the body structure. These alternating portions are amplified and then segregated by sampling devices operating in synchronism with the red/infrared switching, so as to provide separate signals on separate channels representing the red and infrared light transmission of the body structure. After low-pass filtering to remove signal components at or above the switching frequency, each of the separate signals represents a plot of optical transmissivity of the body structure at a particular wavelength versus time.
Because the volume of blood in the body structure varies with the pulsatile flow of blood in the body, each such signal includes an AC component caused only by optical absorption by the blood and varying at the pulse frequency or heart rate of the organism. Each such signal also includes an invariant or DC component related to other absorption, such as absorption by tissues other than blood in the body structure. According to well known mathematical formulae, set forth in said U.S. Pat. No. 4,407,290, the oxygen saturation in the blood can be derived from the magnitudes of the AC and DC components of these signals.
As also set forth in the '290 patent, the same general arrangement can be employed to monitor constituents of the blood other than oxygen such as carbon dioxide, carbon monoxide (as carboxyhemoglobin) and/or blood glucose, provided that the other constituents have some effect on the optical properties of the blood. Also, information concerning the pulse of the patient can be obtained from the AC signal components. As used in this disclosure, the term "parameter of the blood" includes the level of any constitutent and also includes parameters related to the pulse, such as the pulse rate and the occurrence or non-occurrence of pulses.
Measurement apparatus and methods of this type have been widely adopted in the medical profession. However, such apparatus and methods have been subject to interference from ambient light falling on the photodetector. The apparatus has been provided with circuits for cancelling components caused by ambient light. These circuits operate by obtaining a "dark current" signal representing the amplified photodetector signal during intervals when both of the light emitting diodes are off and hence all of the light falling on the photodetector represents ambient light. The dark current signal value is used to cancel the ambient light component in the amplified signals representing infrared and red light.
This approach provides only a partial solution to the ambient light interference problem. The ambient light impinging upon the photodetector may be far stronger than the light transmitted through the patient's body. Accordingly, components of the photodetector signal caused by ambient light may be far larger than the useful photodetector signal components representing light transmitted through the body structure. The ambient light components can overload the first amplifier in the system, commonly referred to as the preamplifier. To avoid such overloading, the gain of the preamplifier has been limited heretofore. The limited gain available in the preamplifier may result in a loss of sensitivity in the instrument as a whole and may require greater gain in subsequent stages used to amplify various portions of the signal.
The conventional preamplifier utilized heretofore incorporates an operational amplifier having inverting and non-inverting input nodes and an output node. The non-inverting input node may be grounded. The photodetector signal, typically a current from a photodiode operating in a photoamperic mode, is connected to the inverting input node of the operational amplifier. A feedback resistor is connected between the inverting input node and the output node. In this "transresistance" amplification arrangement, the operational amplifier creates a voltage at the output node opposite in sense to the voltage at the inverting input node. The opposite sense voltage causes a current flow through the feedback resistor opposite in sense to the current flow applied by the photodetector. The preamplifier comes to equilibrium when the current flow out of the inverting input node through the feedback resistor exactly balances the current flow into the inverting input node through the photodetector. The gain or ratio of output node voltage to incoming signal is proportional to the value of the feedback resistor. The greater the value of the feedback resistor, the greater the opposite sense voltage at the output node must be to achieve balance.
In the typical dark current cancellation circuitry utilized heretofore, the output node of the preamplifier is connected to a first side of a capacitor, whereas the second side of the capacitor is connected to the downstream signal processing equipment. A controllable switch is connected between the second side of the capacitor and ground. The switch is closed only when the light emitting diodes are off, i.e., only during dark intervals. During each dark interval the preamplifier output node voltage represents only the component caused by the ambient light. With the second side of the capacitor grounded, the charge on the capacitor accumulates until the voltage across the capacitor is equal to this voltage. When the dark interval ends, the switch is opened, leaving the charged capacitor connected between the amplifier output node and the downstream signal processing apparatus. Therefore, the voltage applied to the signal processing apparatus will be the preamplifier output voltage less the voltage across the capacitor, i.e., the preamplifier output voltage less the voltage component caused by ambient light. So long as changes in ambient light levels between successive dark intervals are relatively small, this arrangement should theoretically provide good cancellation of the signal components caused by the ambient light.
However, the dark current cancellation afforded in this arrangement does not alleviate the problem of preamplifier overloading. Thus, the operational amplifier must still provide sufficient voltage at the output node so that the current through the feedback resistor completely balances both the useful and ambient-light components of the signal applied to the input node. The value of the feedback resistor, and hence the gain of the preamplifier must be limited to avoid exceeding the capacity of the operational amplifier. Additionally, the operational amplifier is connected directly to a significant capacitive load. Depending upon the design of the particular operational amplifier, the capacitive load may induce instabilities in the operational amplifier.
Accordingly, there have been needs for further improvements in the blood parameter measuring apparatus, and specifically in the ambient light cancellation arrangements used therein.